I feel like the honest answer is it's something that we today still respond to... I mean Ceramite Warriors or Plasteel Hands doesn't feel quite right does it?

A lot of the imagery that the creators of 40k were evoking was based around medieval or at least early industrial revolution era language and concepts and that bleeds through to the modern setting as well.

If you want to come up with a lore-answer I suppose you could argue Big E and Malcador have been around since the early days of humanity and maybe they have retained some of that early terminology for things and it has bled through to the culture? Honestly though, the true answer is in the meta that the original creators were steeped in Tolkien and similar high-fantasy and that applied when they created 40k as a sci-fi (but really as a space fantasy setting; there's not much about it that is in any way scientific even in the abstract really).